Fast phase-frequency detector arrangement

ABSTRACT

The present invention relates to a detector arrangement and a charge pump circuit for a recovery circuit recovering timing information for random data. The detector arrangement comprises a first latch circuit for sampling a quadrature component of a reference signal based on an input signal, to generate a first binary signal, a second latch circuit for sampling an in-phase component of the reference signal based on the input signal, to generate a second binary signal, and a third latch circuit for sampling the first binary signal based on the second binary signal, to generate a frequency error signal. Furthermore, the charge pump circuit comprises a differential input circuit and a control circuit for controlling a tail current of the differential input circuit in response to a frequency-locked state of frequency detector arrangement.

The present invention relates to a detector arrangement for detecting afrequency error between an input signal and a reference signal, and to acharge pump circuit for use in such frequency detector arrangement.Furthermore, the present invention relates to a frequency errordetection method and a charge pump controlling method and to a recoverycircuit comprising the detector arrangement and the charge pump circuit,for recovering timing information from random data.

In recent years there has been a significant research effort in the areaof high-speed electronics for communication. In order to take fulladvantage of the broadband capabilities of optical fibers, there is needof high-speed electronic building blocks, in particular integratedsolutions to reduce costs and improve reliability. Generally, when longdistances are involved, regeneration of data is compulsory since datagets distorted due to fiber impairments on signal transfer. Fiberdispersion, a well known fiber impairment translates into pulse widthdistortion. To guarantee data recovery at reasonable bit-error rate(BER), the signal is typically regenerated after a certain distance (forinstance, in SONET about every 500 km; in 10 Gbit Ethernet, about every10 to 40 km) to prevent degradation of the signal-to-noise ratio (SNR).The regeneration is typically performed in an optical transceiver.

FIG. 1 shows a schematic block diagram of an architecture of an opticaltransceiver. The receiving and transmitting blocks are composed ofseveral analog and digital integrated circuits. At the receiver side,optical signals are converted into electrical signals, data signals areregenerated and, finally, a serial to parallel converter demultiplexesthe retimed serial signal to a lower rate, at which it can be processedby digital circuitry, such as a CMOS (Complementary Metal OxideSemiconductor) circuitry. At the transmitter side, a parallel to serialconverter, where a PLL (Phase Locked Loop) circuitry generates a lowjitter clock for retiming of the NRZ (Non Return to Zero) data stream,multiplexes the parallel data to a high rate serial data, and finallythe serial data signal can be converted back into optical pulses. Ingeneral, the term “jitter” is used here to indicate any kind of randomand/or undesired phase variation.

The data transmitted over the optical fibers is encoded in NRZ format,which implies that no information of the clock signal can be extracteddirectly from the stream due to the fact that the signal does not returnto zero level after each positive or negative bit value. When long datasequences without transition occur, synchronisation at the receiver endbecomes very difficult. This is due to the properties of NRZ data whichspectrum has nulls at frequencies that are integer multiples of the bitrate. For this reason, a nonlinear circuit is needed to recover theclock spectral component from the bit stream.

According to FIG. 1, the transmitting end receives data from a dataprocessor 10 and generates a serial data stream at a multiplexer 20using a PLL circuit 22. The data stream is supplied to a laser driver30, which drives a laser device 40 so as to couple an optical signalinto an optical fiber. The function of the optical receiver is thedetection of the incoming optical NRZ signal and the regeneration orrecovery of the transmitted data. The combination of a photo detector50, and a subsequent transimpedance amplifier 60 is known as the “frontend” portion. The photo detector 50 may be a PIN photodiode or anavalanche photo detector, which convert the optical signals received viathe optical fiber into electrical signals. The low level signalgenerated by the photo detector 50 is amplified by the transimpedanceamplifier 60, which may be a low-noise preamplifier, followed by alimiting amplifier 70 with automatic gain control. A clock extractionand data regeneration circuit (DCR or CDR) 80 recovers the timinginformation from the random data and samples the data stream at anappropriate instant or timing. Finally, a serial to parallel converteror demultiplexer 90 demultiplexes the retimed serial data to a lowerrate, where it can be processed by the digital circuitry of the dataprocessor 10. In the demultiplexer 90, a frame alignment unit 92 may bearranged for synchronization purposes. In order to perform synchronousoperations such as retiming and demultiplexing of random data,high-speed receivers must generate a clock. To achieve this, a clockrecovery circuit senses the data and produces a periodic clock. Data canbe retimed in a D-flip-flop or D-latch by the recovered clock, i.e. itsamples the noisy data, yielding an output with less jitter. Such aflip-flop or latch circuit is sometimes called decision circuit. Therecovered clock should preferably have a frequency equal to the datarate, so that, for example, a data rate of 10 Gb/s translates to a clockfrequency of 10 GHz with a period of 100 ps. Furthermore, the recoveredclock should bear a certain phase relationship with respect to the data,allowing optimum sampling of the bits by the clock. If the rising edgesof the clock coincide with the midpoint of each bit, the sampling occursfarthest from the preceding and following data transitions, providingmaximum margin for jitter and other timing uncertainties. Finally, therecovered clock should exhibit a small jitter since it is the maincontributor to the retimed data jitter. To generate the clock wave form,a voltage controlled oscillator (VCO) is employed which is phase-lockedto the input data using e.g. a flip-flop or latch circuit operating as aphase detector. A low pass filter suppresses ripple on the oscillatorcontrol line. Also, to retime the data, another flip-flop or latchcircuit may be added, which is clocked by the VCO output. Hence, therecovered clock drives the input of the phase detector and the clockinput of the retimer.

FIG. 24 shows a schematic block diagram of a conventional implementationof the CDR circuit in FIG. 1. According to FIG. 24, input data D_(in) issupplied as a clock signal to a first D-flip-flop or latch circuit 802which operates as a phase detector, and to a second D-flip-flop or latchcircuit 808 which operates as a retimer circuit and generates therecovered output data D_(out). The output of the first D-flip-flop 802is supplied via a low pass filter 804 to a VCO 806 which generates arecovered clock signal CK_(out). The recovered clock CK_(out) issupplied as input signal to the first flip-flop circuit 802 and as clocksignal to the second flip-flop circuit 808. This known CDR circuitoperates as follows. Upon turn-on, the first flip-flop circuit 802multiplies the edge-detected input data D_(in) by the output clock ofthe VCO 806, generating a beat that drives the VCO frequency towards theinput bit rate. If the initial difference between the VCO frequency andthe data rate is sufficiently small, the loop locks, establishing awell-defined phase relationship between the input data D_(in) and therecovered output clock CK_(out). In fact, with such a bang-bangcharacteristic provided by the first flip-flop circuit 802 as phasedetector, the data edges settle around the zero-crossing points of theclock. Even for a slight phase error, the first flip-flop circuit 802generates a large output, driving the loop towards the locking state.

However, the known CDR circuit of FIG. 24 suffers from a number ofdrawbacks. First, the first flip-flop circuit 802 may produce fulldigital outputs for run lengths greater than one, thereby creatingsubstantial ripple on the control voltage of the VCO 806 and hencejitter at the output. Second, since the first flip-flop circuit 802samples the clock by the data, whereas the second flip-flop circuit 808samples the data by the clock, data retiming exhibits significant phaseoffset at high speeds. Typically, flip-flops display unequal delays fromthe D-input to the output and from the clock input to the output. Thus,if for example the CK-to-Q delay is longer than the D-to-Q delay by ΔT,the first flip-flop 802 locks such that the data leads the clock by ΔT,sampling the clock closer to the zero crossing after the dataexperiences the intrinsic delay of the first flip-flop circuit 802.

The output of the VCO 806 suffers from even more delay as it propagatesthrough the second flip-flop circuit 808, sampling the data far from themiddle of the eye generated by superposition of several signal periods.In other words, if the difference between the CK-to-K and D-to-Q delaysis equal to ΔT, the retiming suffers from a skew or delay of 2ΔT. Third,the simple CDR architecture of FIG. 24 relates to the feedthrough ofdata to the VCO output through both flip-flop circuits 802, 808. Theoutput phase is disturbed on arrival of each data transmission,requiring that the VCO 806 be followed by a buffer stage providingsignificant reverse isolation.

In general, CDRs for NRZ data can be grouped into open loop and closedloop circuits. The clock recovery circuits limit the obtainable datarate of multi-gigabit-per-second integrated fiber optic receivers. It isby far the most complicated building block of the transceiver and themost difficult to design.

FIG. 25 shows a schematic block diagram of a conventional CDR circuitwith two separate loops and autonomous frequency locking on the randominput data D_(in). The presence of the two loops provides thepossibility to dimension the loops in different ways and to achieve thespecifications by decoupling the two opposite requirements of CDRs, i.e.fast frequency acquisition and low jitter in the locked state. Thefrequency loop (FL) is dimensioned for fast frequency acquisition whilethe phase loop (PL) is dimensioned for the lowest possible jitterpeaking. When data transitions are present, a frequency detector 811acquires the correct frequency and gives a zero DC signal as coarsesignal V_(CO) to a coarse input of a VCO 817, which generates therecovered clock signal CK_(out) which is also fed back to the frequencydetector 817 and to a phase detector 822 of the phase loop. The phaseerror signal is supplied as fine signal V_(fi) to a fine input of theVCO 817 via similar charge pump and low-pass filter circuits 823, 825.The random input data D_(in) is supplied to the frequency detector 811and to the phase detector 822 via a limiter circuit 819 which provides alimiting operation to the amplitude of the random input data. The phasedetector 822 may be a bang-bang phase detector.

With the coarse-fine-loop idea, the fine loop is for the phase and theother coarse loop is for the frequency. When the frequency difference islarge, the phase loop can be considered open since its gain is verysmall. The frequency loop gain can be increased independent from thejitter so that the pull-in range can be increased without increase ofthe jitter. However, a problem arises from the fact that the two loopsare always active. Due to this, the frequency detector 811 may add extrajitter after the frequency-locked state has been reached.

Phase detectors and frequency detectors for random high-speed NRZ datahave the difficult task to work on random transitions of the dataBetween transitions, the phase and frequency detection should keep thephase and frequency information such that the VCO is not pulled awayfrom the locked state when transitions are missing. Although, in theknown solutions for digital implementations of frequency detectors, analmost zero DC input can be derived in the locked state by usinglow-pass filters, the bang-bang nature of the frequency detectorsenhances the jitter generated by the VCO, making them impractical forlow-jitter systems.

It is therefore an object of the present invention to provide animproved detector circuit, which can be used in low-jitter systemsoperating at high frequencies.

This object is achieved by a detector arrangement for detecting afrequency error between an input signal and a reference signal thatincludes first latch means for sampling a quadrature component of thereference signal based on the input signal to generate a first binarysignal, second latch means for sampling an in-phase component of thereference signal based on the input signal to generate a second binarysignal, and third latch means for sampling the first binary signal basedon the second binary signal to generate a frequency error signal. Thisobject may also be achieved by a charge pump circuit for use in afrequency detector arrangement that includes a differential inputcircuit having first and second differential branches, modulating meansfor modulating first and second current sources respectively arranged inthe first and second differential branches, and control means forcontrolling a tail current of the differential input circuit in responseto a frequency-locked state of the frequency detector arrangement. Thisobject may also be achieved by a method of detecting a frequency errorbetween an input signal and a reference signal that includes the stepsof sampling a quadrature component of the reference signal based on theinput signal to generate a first binary signal, sampling an in-phasecomponent of the reference signal based on the input signal to generatea second binary signal, and sampling the first binary signal based onthe second binary signal to generate a frequency error signal. Thisobject may also be achieved by a method of controlling a charge pumpcircuit used in a frequency detection arrangement that includes thesteps of modulating first and second current sources respectivelyarranged in first and second differential branches of a differentialinput circuit of the charge pump circuit and controlling a tail currentof the differential input circuit in response to a frequency-lockedstate of the frequency detector arrangement.

Accordingly, the claimed combination of latch means providestransformation of the phase difference between the input signal and thereference signal into positive or negative binary signals. When thesignal is positive, the reference signal, e.g. clock signal, willincrease its phase, and for negative signals, the reference signal willdecrease its phase. The two binary quadrature signals are used in thethird latch means in order to produce a frequency error signal. Once thefrequency-locked state is acquired, the output of the frequency detectorgives a zero DC signal at the output such that the subsequent VCO keepsthe frequency information. Due to its simplicity, this principle can beused in high-speed clock and data recovery systems, such as opticalcommunication systems.

Furthermore, the proposed differential charge pump arrangement providesthe advantage of being less sensitive to common-mode noise from thepower supplying lines and substrate. The principle of controlling thetail current of the charge pump provides the advantage that it will beactive only in a frequency error from the detector arrangement needs tobe processed. Thus, the charge pump circuit will be switched offcompletely in the locked state, which leads to a reduced jitter in therecovery system.

Furthermore, control means may be provided in the detector arrangement,for selectively suppressing operation of the charge pump circuit towhich the first binary signal is supplied, in response to a controlsignal derived from the second binary signal.

The first and second latch means of the detector arrangement each maycomprise a double-edge triggered flip-flop arrangement. As an example,the double-edged triggered flip-flop arrangement may comprise first andsecond D-latch circuits receiving the input signal and beingrespectively controlled by a direct version and an inversed version ofthe respective component of the reference signal, and a multiplexercircuit being controlled by the inversed version of the respectivecomponent. This double-edge detector arrangement is thus clocked on bothtransitions of the random input data signal, wherein the in-phasecomponent and the quadrature phase component are two full-speed clocksignals. The output of the multiplexer circuit can thus be updated onlyat transitions of the input signal. Before the next data transitionarrives, the output of the multiplexer circuit is kept on its previousvalue. The third latch means may comprise a D-latch circuit receivingthe first binary signal and being controlled by the second binarysignal. Using such an arrangement, positive to negative transitions ofthe in-phase vector can be monitored, and a frequency error signal canbe generated from the quadrature vector.

The first and second binary signals may be supplied to the charge pumpcircuit via at least one of respective amplifier and level shiftercircuits. In particular, the amplifier circuits each may comprise acombination of a feedback amplifier and a feed forward amplifier.Thereby, signal quality can be improved and the signal swing and/orcommon mode levels can be adapted to suite the need of the next circuitstage, e.g. the charge pump circuit. The combination of the feedbackamplifier and feedforward amplifier provides the advantage ofsuperposition of gain effects, while the output impedance can be reducedand gain can be well controlled by the feedback ratio of the feedbackamplifier.

The charge pump circuit may comprise first and second current mirrorcircuits respectively provided in the first and second differentialbranches and arranged to copy the modulated current of one differentialbranch into the respective other differential branch. Thereby, a doubleoutput variation can be achieved for each branch due to the fact thatthe current and modulation is copied into the respective other branch soas to double the output current supplied to a filter of the charge pumpcircuit.

Furthermore common mode rejection means may be provided for comparing acommon-mode voltage at the output of the charge pump circuit with areference voltage, and for controlling the first and second currentsources based on the comparison result. This provides the advantage thatthe common mode level at the output of the charge pump circuit isconstant irrespective of changes in temperature and supply, and animproved stability of the oscillation frequency of the VCO in therecovery loop can be achieved.

The control means of the charge pump circuit may comprise switchingmeans for switching the tail current in response to a control signalindicating the frequency-locked state. Thereby, the tail current can beshut down completely so that there will be no jitter contribution e.g.from the coarse loop in which the charge pump circuit with therespective detector arrangement is used.

The present invention will now be described in greater detail based onpreferred embodiments with reference to the accompanying drawings, inwhich:

FIG. 1 shows a schematic block diagram of an optical transceiver inwhich the present invention can be implemented,

FIG. 2 shows a digital implementation of a quadricorrelator on which thedetector arrangement according to the preferred embodiments is based,

FIG. 3 shows a frequency detector arrangement with combined charge pumpcircuit according to the first preferred embodiment,

FIGS. 4A and 4B show waveforms for a double-edge latch-multiplexerconfiguration for an early clock signal and a late clock signal,respectively,

FIG. 5 shows a schematic block diagram for generating two quadraturesignals at full rate,

FIG. 6 shows a vector representation of an equilibrium position of I andQ vectors in a phase-locked state,

FIG. 7 shows vector diagrams for visualizing a frequency detectionalgorithm according to the preferred embodiments,

FIG. 8 shows a D latch circuit in Source Coupled Logic,

FIG. 9 shows a multiplexer circuit in Source Coupled Logic,

FIG. 10 shows a schematic block diagram indicating the principle of adifferential charge pump circuit according to the first preferredembodiment,

FIG. 11 shows a schematic block diagram indicating a common-moderejection principle, which can be used in the charge pump circuitaccording to the preferred embodiments,

FIG. 12 shows a schematic circuit diagram of an implementation of thedifferential charge pump circuit according to the preferred embodiments,

FIG. 13 shows a schematic circuit diagram of a common-mode sensecircuit, which can be used in the differential charge pump circuitaccording to the preferred embodiments,

FIG. 14 shows a schematic circuit diagram of a differential charge pumpcircuit with a common-mode circuit according to the preferredembodiments,

FIG. 15 shows a schematic circuit diagram of a charge pump circuit witha tri-state control circuit according to the preferred embodiments,

FIG. 16 shows a schematic block diagram of a detector arrangement with acharge pump circuit according to a second preferred embodiment,

FIG. 17 shows a schematic circuit diagram of an amplifier circuit withlevel shift function according to the second preferred embodiment,

FIGS. 18A and 18B show respective schematic circuit diagrams ofequivalent circuits of a feedback amplifier and a feed forward amplifierof the amplifier circuit of FIG. 17,

FIGS. 19A and 19B show schematic circuit diagrams of level shiftercircuits for the in-phase arm and the quadrature arm of the secondpreferred embodiment,

FIG. 20 shows a waveform diagram indicating phase detectioncharacteristics of the second preferred embodiment,

FIG. 21 shows a schematic circuit diagram of a tri-state circuit of thepreferred embodiments,

FIG. 22 shows a waveform diagram indicating a phase detectorcharacteristic according to the preferred embodiments,

FIG. 23 shows a waveform diagram indicating a frequency detectorcharacteristic of the second preferred embodiment,

FIG. 24 shows a schematic block diagram of a known clock extraction anddata regeneration circuit,

FIG. 25 shows a schematic block diagram of a known clock extraction anddata regeneration circuit with two loops and autonomous frequencylocking, and

FIG. 26 is a process flow diagram of a method of controlling a chargepump circuit used in a frequency detection arrangement in accordancewith an embodiment of the invention.

The preferred embodiments will now be described on the basis of afull-rate phase-frequency detector based on a quadricorrelator circuit.

Frequency discriminators based on quadricorrelator circuits aredescribed for example in H. Ransijn and P. O'Connor, “A PLL-based2.5-Gb/s GaAs clock and data regenerator IC.” IEEE J. Solid-StateCircuits, vol. 26(10), pp. 1345-1353, October 1991, R. R. Cordell et al.“A 50 MHz phase and frequency-locked loop.” IEEE J. Solid-StateCircuits, vol. 14(6), pp. 1345-1353, October 1991, and B. Razavi and J.Sung, “A 2.5 Gb/s 15-mW BiCMOS Clock Recovery Circuit”, Symposium onVLSI Circ. Dig. Of Tech. Papers, pp. 83-85, 1995. Furthermore, an analogimplementation of a quadricorrelator concept based on sample and holdcircuits has been described in A. Pottbacker et al., “A 8 Gb/s Sibipolar phase and frequency detector IC for clock extraction”, ISSCCDig. Tech. Papers, pp. 162-163, San Francisco, Calif. February 1992.

According to the preferred embodiments of the present invention, adigital detector arrangement based on a digital implementation of thequadricorrelator concept is presented.

FIG. 2 shows a schematic circuit diagram of a digital implementation ofthe quadricorrelator, on which the preferred embodiments are based. Thisdigital implementation comprises three latch-multiplexer configurationsconsisting of two D-latches and one multiplexer. Each latch-multiplexerconfiguration corresponds in function to a double-edge flip-flop circuitand could be replaced by such a flip-flop circuit. This double-edgeflip-flop circuit is clocked on both transitions of the input datasignal DATA. Two full-speed clock signals CKI and CKQ are in quadraturewith each other, and thus can be regarded as an in phase clock componentCKI and a quadrature clock component CKQ. The term “full-speed”indicates that one period of the clock signal corresponds to the bitlength Tb of the input data signal.

The double-edge flip-flop function acts to sample both clock componentsCKI and CKQ at transitions of the input data signal, providing twoquadrature signals PT_Q and PT_I. The output of the multiplexer MUX willthus be updated only at transitions of the input data signal DATA.Before the next data transition arrives, the output of the multiplexerMUX is kept on the previous value. The phase difference between theinput data signal DATA and the clock components CKQ and CKI,respectively, is transformed into a positive or negative quantized orbinary signal. When this signal is positive, the clock will increase itsphase, and when the signal is negative, the clock will decrease itsphase.

In the following, for the sake of simplicity, the two quadrature outputsPD_Q and PD_I are denoted as Q and I, respectively. The thirdlatch-multiplexer configuration functions as a third double-edgeflip-flop circuit which samples the output signal or vector Q at thetransitions of the output signal or vector I. Thus, the frequency errorcan be obtained at the output of the third latch-multiplexerconfiguration. This frequency error is also obtained in a quantized orbinary form. Therefore, in a phase locked situation, the outputfrequency error signal ripples between a positive and negative valuewith a period corresponding to the bit period Tb of the input datasignal and a 50% duty cycle.

It is noted that in the third latch-multiplexer configuration comprisingthe D-latches L5 and L6, a sign inversion is introduced at the output ofthe upper D-latch L5, which leads to an inversion of the Q vector atpositive values of the I vector. This inversion is needed to implement adifferentiator function of the analog quadricorrelator which givespositive values on the rising edge and negative values on the fallingedge.

The equilibrium or balanced position for the vectors I and Q can berepresented with the rotating wheel analogy in a vector diagram, asshown in FIG. 6. According to FIG. 6, the vector I is positive, stableand equal to “+1” whilst the vector Q bounces from the positive to thenegative quadrant in a periodic fashion due to the bang-bangcharacteristic of the circuit. This frequency error generation can beexplained by using the rotating wheel analogy as follows. When the clockis too slow, the pair of the two quadrature vectors I and Q rotatescounter-clockwise with an angular frequency equal to the frequencydifference Δω and the derivative of the vector I falls before the vectorQ, generating an error signal. On the other hand, when the clock is toofast, the pair of two quadrature vectors I and Q rotates clockwise withan angular frequency equal to the frequency difference Δω and thederivative of the vector I falls before the vector Q with 180° phasedifference, generating an error signal.

FIG. 3 shows a schematic block diagram of a detector arrangement withcharge pump circuit according to the first preferred embodiment. Thisdetector arrangement is based on the bang-bang principle and works atfull-rate with the two clock components CKQ and CKI in quadrature witheach other. The random transitions on the input data signal DATA areused to sample the clock to generate a phase error signal. Twoquadrature phase error signals PD_I and PD_Q are generated at the sametime. The two quadrature phase error signals PD_Q and PD_I areconsequently used in order to produce a frequency error signal FD. Oncethe frequency locked state is acquired, the frequency detector gives azero DC voltage at the output so that the subsequent VCO keeps thefrequency information. Due to its simplicity, this principle can be usedin high-speed clock and data recovery, as for example in opticalcommunication systems.

In particular, the detector arrangement in FIG. 3 comprises twobang-bang phase detectors, which output signals are in quadraturerelation with each other, and are respectively called PD_Q and PD_I.

In case of autonomous locking, the frequency information needs to berecovered from the transitions of the input data signal DATA. In thiscase, a full-rate frequency detector with tri-state output or ahalf-rate frequency detector with tri-state output can be used. In bothcases, an extra output is obtained as a bang-bang phase detector output.FIG. 3 depicts the full-rate frequency detector which has two quadratureclock input signals CKQ and CKI at full speed, i.e. f_(clk)=1/Tbit,which means that the clock frequency f_(clk) corresponds to the maximumbit rate of the input data signal DATA.

In order to understand the operation of the phase-frequency detectorarrangement of FIG. 3, the latch-multiplexer combination having adouble-edge triggered flip-flop function is described in more detail.The latch circuits L1 to L4 sample the respective clock signal with theinput data signal DATA on both up and down going edges of the input datasignal DATA. The upper latch circuit L1, L3 is transparent to therespective clock input when the input data signal DATA is high. Thelower circuit L2, L4 is transparent to the respective clock input whenthe input data signal DATA is low. The speed of operation of thisdouble-edge latch-multiplexer configuration is high since the two upperand lower latch or sampling circuits work interleaved. When the upperlatch circuit samples the clock signal, the lower latch circuit deliversits output, and when the lower latch circuit samples the clock signal,the upper latch circuit delivers its output. The output of themultiplexers MUX will be updated only at data transitions, while betweenthe data transitions the output value is kept constant. As aconsequence, the double-edge triggered latch-multiplexer configurationgive sample versions of the clock signal. At data transitions, theoutput of the latch-multiplexer configurations will follow the clocktransitions. Therefore, the double-edge triggered latch-multiplexerconfiguration can be used as a phase detector, wherein the phasedifference between the input data signal DATA and the clock componentsCKQ and CKI is transformed into respective quantized or binary signals.

FIGS. 4A and 4B show waveforms relating to the double-edgelatch-multiplexer configuration, wherein FIG. 4A relates to the casewhere the clock signal is early, i.e. it advances the input data signal,and FIG. 4B relates to the case where the clock signal is late, i.e.follows the input data signal. If the respective output voltage V_OUT ofthe latch-multiplexer configuration is positive, the respective clocksignal is early and needs to decrease its phase. If the respectiveoutput voltage V_OUT is negative, the clock signal is late and needs toincrease its phase. As can be gathered from FIGS. 4A and 4B, each of thelatch-multiplexer configuration has a bang-bang characteristic, i.e. theoutput voltage V_OUT is either positive or negative depending on thefact whether the respect clock signal is early or late.

As a frequency information is required from the transitions of the inputdata signal DATA, more clock phases at full-rate are needed, such thattwo double-edge latch-multiplexer configurations are used to generatethe two quadrature phase error signals PD_Q and PD_I.

FIG. 5 shows such a double-edge latch-multiplexer configuration forgenerating two quadrature signals Vi and Vq based on two clock signalsor clock components CKI and CKQ in quadrature, which are sampled by theinput data signal DATA. From these two quadrate phase error signals Viand Vq, the frequency error signal can be extracted as follows.

The data transitions sample the two quadrature clock components CKI andCKQ at full speed. In FIG. 3, the phase detection outputs PD_Q and PD_Icorrespond to the outputs of two phase detectors and are in quadraturewith each others, which means they have a phase difference of 90°. Inthe following, again, the two quadrature phase outputs PD_Q and PD_I aredenoted as Q and I outputs, respectively.

As already explained, FIG. 6 shows the equilibrium position for thecorresponding vectors I and Q in the rotating wheel analogy. In thelocked phase state, the vector I is positive, stable and equal to “+1”whilst the vector Q bounces from the positive to a negative quadrant ina periodic fashion. By monitoring the positive to negative transitionsof the vector I, the frequency error can be obtained based on thefollowing algorithm:

-   1. When the vector I has a negative to positive transition, which    means it changes from a negative to a positive quadrant in the    vector diagram, the frequency must be kept by generating a zero    signal at the output of the frequency detector.-   2. When the vector I has a negative to positive transition, which    means that it changes from a negative quadrant to a positive    quadrant, for negative vectors Q, the frequency must be kept by    generating a zero signal at the output of the frequency detector.-   3. When the vector I has a positive to negative transition and the    vector Q is positive, the frequency has to be increased by    generating a frequency error signal FD=+1.-   4. When the vector I has a positive to negative transition and the    vector Q is negative, the frequency has to be decreased by    generating a frequency error signal FD=−1.

FIG. 7 shows a visualization of this algorithm based on different vectordiagrams indicating the above four cases. In operation, each of the fourpossible cases will converge towards the equilibrium position shown inFIG. 6. In the following table, the four cases or situations arepresented, which can be used to construct the logic for the frequencydetector:

PD_I (I vector) PD_Q (Q vector) FD −/+ −1 0 −/+ 1 0 +/− −1 −1 +/− 1 +1

As shown in FIG. 3, the vector I is being used to clock a latch circuitL5 which samples the vector Q (which corresponds to the phase detectionoutput PD_Q). Furthermore, as depicted in FIG. 3, the other phasedetection output PD_I and its inverse output are sign-inverted (SI) andsupplied to respective transistors M1 and M2 of a differential tri-statecontrol circuit TS which controls a charge pump circuit 82. Based on thevalue of the vector I (phase detection output PD_I), the tail currentI_(bias) of the differential control circuit flows through the sourcesof another differential pair M3, M4 or can be dumped or forced to apredetermined current I_(—)3State. The vectors Q and I correspond to thequantized phase error wherein the vector I is used to sample the vectorQ with the latch circuit L5. When the vector I is positive, the latchcircuit L5 is transparent to the vector Q, but the tri-state controlcircuit TS is not transparent to the output signal FD of the latch L5.This means that no frequency error will be generated when the vector Iis positive. Only when the vector I changes from positive to negativevalues, the tri-state control circuit TS starts to be transparent to theoutput of the latch circuit L5 and at this moment the latch circuit L5latches the last sampled value, i.e. the latest value of the vector Q.In equilibrium, when the vector I is positive and “+1”, the otherdifferential pair M3, M4 is not active any longer and the differentialoutput of the frequency detector VFD is zero. The dump currentI_(—)3State from the transistor M2 can be used to switch off the chargepump circuit 82.

An advantage of this implementation resides in the possibility ofpull-in with large frequency errors leading to an improved frequencydetector arrangement due to its simplicity and frequency error detectioncapabilities. However, due to the fact that the phase detectorarrangement generates a quantized version of the phase error extraripple is generated in the phase-locked state at the fine input of theVCO. Nevertheless, this extra ripple can be reasonable small. The amountdepends on the amount of data transitions per time unit. More datatransitions mean that the frequency detector has more information andthus the VCO frequency will not drift too far away from the lockedfrequency. However, using a tri-state charge pump circuit can alleviatethis extra ripple, as explained later.

In the following, practical implementations of the latch circuits L1 toL5 and the multiplexer circuits MUX are described in more detail.

FIG. 8 shows a schematic circuit diagram of a D-latch circuit in SourceCoupled Logic (SCL) which can be used as latch circuits L1 to L5. ThisD-latch circuit is based on a CMOS (Complementary Metal OxideSemiconductor) Current Mode Logic (CML) which is a CMOS implementationof the bipolar Emitter Coupled Logic (ECL). Similar to ECL, signalswings can be made small by this technology. A bias current I_(bias)flows in the sources of the CMOS transistors M5, M6 or M3, M4, independence on the polarity of the clock signal CLK. Furthermore, thepolarity of the data signal D selects the current path to the output Qwhen the data signal D is negative, or to the output Q when the datasignal D is positive. Thereby, the function of the latch circuits L1 toL5 can be obtained.

FIG. 9 shows an implementation example for the multiplexer circuit MUXin SCL. Similar to FIG. 8, a bias current I_(bias) flows in the sourcesof the CMOS transistors M5, M6 or M3, M4, independence on the polarityof a selection signal S. The polarity is of the signals Ch1, Ch2 selectthe paths for the current. When the selection signal S is positive, thebias current flows in the source of the CMOS transistors M3 and M4. Whenthe selection signal S is positive and the channel signal Ch1 ispositive, then the output Q is positive, and the bias current will flowthrough a resistor R1 and the transistor M3. When the selection signal Sis positive and the channel signal Ch1 is negative, then the output Q isnegative, and the bias current will flow through another resistor R0 andthe transistor M4. Thus, when the selection signal S is positive, themultiplexer circuit is transparent to the channel signal Ch1, whichmeans that the output of the multiplexer is equal to the channel signalCh1. Similarly, when the selection signal S is negative, the biascurrent flows in the source of the transistors M5 and M6. Then, themultiplexer circuit is transparent to the second channel signal Ch2,which means that the output of the multiplexer circuit is equal to thechannel signal Ch2.

In the following, a tri-state charge pump circuit is described ingreater detail, which provides the advantage of reduced ripple in theequilibrium state. In particular, this tri-state charge pump circuit canbe used as the charge pump circuit 82 in FIG. 3.

Phase or frequency comparison at high speed requires fast charge pumpcircuits for filtering the spurious signals at the output of thephase/frequency detector arrangement. In clock and data recovery, thecomparison between the transitions of the input data signal DATA and theclock transitions is done at full speed since frequency dividers cannotbe used due to the random character of the input data signal DATA.

Therefore, a differential charge pump circuit is proposed which can beused for high-speed phase/frequency detectors. A differential chargepump has the advantage of being less sensitive to common-mode noise fromthe power supply lines and substrate. In general, a charge pump circuitis a current source which charges/discharges a filter inside thefrequency and phase loops depicted in FIG. 25. The input of the chargepump circuit can be a fast series of pulses, so that it has to beensured that a fast switching between both states and a linearrepresentation of the phase error in the form of a charge in thecapacitor of the loop filter is obtained. The simplest implementation ofthis principle is a circuit which has two digital input lines to drivetwo switches which provide a constant charging or discharging current toa capacitor. Then, the output voltage at the capacitor corresponds tothe integral of the binary signals which drive the respective switches.When both switches are open, the voltage on the capacitor remains thesame. Maintaining the voltage on the capacitor is equivalent to keepingthe same frequency at the output of the VCO.

However, as mentioned above, differential circuits have a betterimmunity towards supply and substrate noise due to the strong rejectionof the common mode noise. Also, the practical implementation of thecapacitor in the single ended solution gives extra parasiticcapacitances in parallel with the capacitor of the filter. In thedifferential mode, however, the capacitor of the filter can be laid outsuch that the parasitic capacitance obtained by circuit lines andcircuit elements is not a part of the differential filter.

FIG. 10 shows a schematic block diagram of a differential charge pumpcircuit as proposed according to the first preferred embodiment. Thebasic idea is to have control on the charge pump circuit in such amanner that it will be active only when the frequency error signal FD atthe output of the frequency detector needs to be processed. This can berealized by controlling the tail current of the differential charge pumpcircuit. The frequency information may be differentially pumped into theoutput filter of the charge pump circuit.

According to FIG. 10, a differential input signal Δx is adapted tomodulate current sources I₀ in both branches of the differentialcircuit. To achieve a double output variation, for each branch, the DCcurrent and the modulation are copied into the upper part of the otherbranch via two respective current mirror circuits CM having a mirrorratio 1:1.

This charge pump circuit leads to the advantages that the double outputcurrent is supplied to a filter F, compared to the traditionaldifferential charge pump, and an on-off control can be achieved bycontrolling the tail current of the charge pump circuit.

Furthermore, rejection of common-mode signals from the supply lines andsubstrate is improved due to differential implementation. As thecommon-mode level influences the oscillation frequency of the subsequentVCO, enhanced control of the common-mode level of the charge pumpcircuit may be necessary. To achieve this, a common-mode rejectioncircuit can be used to control the common-mode level at the output ofthe charge pump circuit, so that the output common mode will be constantirrespective of changes in temperature and supply.

FIG. 11 shows an improved implementation example of the charge pumpcircuit with an additional common-mode rejection circuit. In particular,the common mode at the output is rejected with a negative feedbackcircuit C which compares the common-mode voltage at the output with aband gap-referenced voltage V_(CM). This common-mode rejection circuit Cadds/subtracts from both current sources the same current as obtained bythe comparison.

FIG. 12 shows a transistor level implementation of the charge pumpcircuit shown in FIG. 10. The input currents I₀+Δx and I₀−Δx comprise aDC current I₀ and a modulation pulsed current Δx. These input currentsare cross-mirrored in the current-mirror transistors M5 and M8,respectively. In this way, the signal Δx is doubled in the filter Fwhich consists of two serial capacitors 2C. Signal current 2Δx flows inthe output filter F, where the capacitor C is represented as the seriesconnection of the two capacitors 2C. The plate of the capacitor 2C withlargest parasitics is connected to the common-mode of the twocapacitors. In this way, the parasitic capacitances are not part of thefilter.

FIG. 13 shows an implementation of a common-mode sense circuit. Thecommon mode at the output of the charge pump circuit, e.g. output nodesOUT+ and OUT− in FIG. 12, can be measured with the transistors M3 and M4in FIG. 13, which work as controlled resistors in a triode regime. TheDC current 4I₀ is forced to a source degenerated mirror circuitconsisting of the transistors M1 and M2. The controlled transistors M5and M6 have their gates connected to the band gap referenced voltageV_(CM). Thus, the mirror ratio at the output of the transistor M2depends on the ratio of the MOS resistors present on the sources of thetransistors M1 and M2. The resistance of the MOS combination depends onthe common-mode voltage as long as the band gap referenced voltageV_(CM) is larger than the threshold voltage of the transistors. Bysubstracting the DC current at the output of the transistor M2 and M3,only the error signal±ΔI will be transmitted at the output of the sensecircuit. The error signal is thus a measure of the difference betweenthe common-mode voltage at the output and the band gap referencedvoltage V_(CM). In the equilibrium state, the sum of the voltages at thenodes or terminals OUT− and OUT+ is double the value of the band gapreferenced voltage V_(CM).

FIG. 14 shows a transistor level schematic circuit diagram of a chargepump circuit with common-mode control as explained above. Theparticularity of this circuit consists of the fact that the signal ismirrored together with the common-mode error signal in order to correctfor the I₀ DC common-mode current. As mentioned above, the charge pumpcircuit can be used in the coarse loop for frequency correction. Whenthe locked state has been reached, this charge pump circuit will shutdown completely so that there will be no jitter contribution from thecoarse loop shown in FIG. 25. This is achieved by controlling the tailcurrent of the charge pump circuit using the intermediate tri-statecontrol circuit TS of FIG. 3. However, in order to use the tri-statecontrol circuit TS to switch the charge pump circuit, an interfacecircuit is needed between the dump current I_(—)3State and the tailcurrent of the charge pump circuit 82.

FIG. 15 shows a schematic circuit diagram of the charge pump circuitaccording to FIG. 14 with a corresponding interface to the dump currentI_(—)3State of FIG. 3.

The dump current I_(—)3State is present or active when the frequency isin the locked state, i.e. the phase detection signal PD_I has the value“+1”. In this case, the biasing current 4I₀ flows through the transistorM3*. The transistor M4* is blocked. Thus, no current is copied to thetail of the charge pump circuit and the charge pump circuit is switchedoff. As soon as a frequency error occurs and the phase detection signalPD_I changes to the value “−1”, the dump current I_(—)3 State isswitched to zero by the tri-state control circuit in FIG. 3, and thebiasing current 4I₀ flows through the transistor M4*. Then, thetransistor M3* is blocked. The same biasing current 4I₀ is copied to thetail of the charge pump circuit is switched on.

As an example, the value of the band gap referenced voltage V_(CM) maybe set to about 1.25V, based on the output common mode of the chargepump circuit.

As mentioned above, the common mode at the output of the charge pumpcircuit is rejected with a negative feedback circuit which compares thecommon-mode voltage at the output with this band gap referenced voltageV_(CM).

FIG. 16 shows a detector arrangement with charge pump circuit accordingto a second preferred embodiment.

In a second preferred embodiment, amplifier circuits 84 have been addedto the first preferred embodiment of FIG. 3 to amplify the signals fromthe double-edge latch-multiplexer configurations. Furthermore, levelshift circuits 86 have been introduced to adapt the common-mode levelsat the data inputs D and clocked inputs CK of the latch circuit L5.Thereby, improved in/out compatibility between the building blocks canbe achieved, to thereby improve signal quality and adapt signal swingand/or common mode levels to suit the need of the next circuit stage.

It is difficult to get the required amplification at the output of abang-bang phase detector constructed, for example, with CMOS18technology. The latch circuits L1 to L4 do not recover fully theamplitude of input signals. The attenuated phase detection outputs PD_Iand PD_Q will thus lead to difficulties in processing the signals in thefollowing stages. The latch circuit L5 is used to sample the quadraturephase detection output PD_Q the in-phase detection output PD_I. As theswing or level of the in-phase detection output PD_I is not big enough,the two transistors MI and M2 of the tri-state control circuit in FIG. 3and FIG. 16 may not be able to get fully imbalanced. Therefore, in orderto achieve enough gain, the additional amplifier circuit 84 is neededbetween the in-phase detection output PD_I and the latch circuit L5. Asimilar amplifier 84 is added between the quadrature detection outputPD_Q and the latch circuit L5, not only to achieve a bigger swing butalso a better delay matching.

FIG. 17 shows a schematic circuit diagram of the amplifier 84. Inparticular, the amplifier 84 consists of a feedback amplifier withresistors R1 and R2 and a differential transistor pair M2 and M3.Furthermore, a feedforward amplifier consisting of the resistor R1, adifferential transistor pair M1 and M4, a load R and transistors M6 andM7 is provided. The feedforward amplifier amplifies the signals at thegates of the transistors M1 and M4, cross-injecting more signal at theoutput through the transistors M6 and M7. The output current will bedelivered by both, the feedforward amplifier and the feedback amplifier.

FIGS. 18A and 18B show equivalent circuits for AC signals of thefeedback amplifier and the feedforward amplifier, respectively. Theoutput voltage Vout of the equivalent circuits of the feedback andfeedforward amplifiers can be estimated by applying superposition.

The output voltage Vout1 of the feedback amplifier of FIG. 18A can beapproximately calculated as follows:

$V_{{out}\; 1} \approx {{- \frac{R\; 2}{R\; 1}} \cdot \frac{V_{id}}{2}}$wherein V_(id) indicates the differential input voltage.

Based on the transconductance gm of the transistors, the output voltageVout2 of the feedforward amplifier circuit of FIG. 18B can beapproximately calculated as follows:

$V_{{out}\; 2} \approx {{- \frac{V_{id}}{2}} \cdot \lbrack ( \frac{R_{L}}{2{R_{1}( {1 + \frac{1}{g_{m}R}} )}} ) \rbrack}$

According to the superposition principle the output voltage Vout of FIG.17 corresponds to the sum of Vout1 and Vout2. As the output voltage inFIG. 17 is obtained at a single side with respect to ground, the outputvoltage of the amplifier circuit corresponds to half of the differentialoutput voltage V_(out) which can be approximately calculated as follows:

Under the assumption g_(m) and R>>1, the voltage gain of the amplifiercircuit 84 of FIG. 17 can be expressed as follows:

$V_{out} \approx {{- \frac{V_{id}}{2}} \cdot \lbrack {( \frac{R_{2}}{R_{1}} ) + \frac{R_{L}}{2{R_{1}( {1 + \frac{1}{g_{m}R}} )}}} \rbrack}$$A_{v} \approx {{- \frac{R_{2}}{R_{1}}} - \frac{R_{L}}{2R_{1}}}$

By adding the feedback amplifier portion, the output impedance isreduced, and the output of the amplifier fits higher capacitance loads.In the present case, this capacitance load is generated by thegate-source capacitances of the next stage. Furthermore, due to thefeedback resistor network, the gain can be better controlled dependingon the resistor ratios.

Considering the structure of the frequency detector in FIG. 16, thecommon mode of both phase detection outputs PD_I and PD_Q from thedouble-edge latch-multiplexer configurations can be estimated to about1.6 V. Thus if the double-edge output is directly connected the levelshift circuits 86, the transistors M1 and M2 of the tri-state controlcircuit would be pushed into their linear region. In order to get acorrect input common mode for the level shift circuits 86, the outputcommon mode of the double-edge latch-multiplexer configuration should beshifted to about 1.2 V. Hence, the amplifier circuit 84 should bedesigned to achieve linear amplification of the output of thedouble-edge latch-multiplexer configuration and to also shift the commonmode level to about 1.2 V needed by the level shift circuits 86.Thereby, the gain can be improved to achieve enough swing so that fullimbalance can be ensured at the tri-state control circuit TS.Furthermore, a common mode level shift necessary for the next stage,i.e. the level shift circuit 86, can be achieved.

FIGS. 19A and 19B show schematic circuit diagrams of the level shiftcircuits 86 on the PD_I arm and the PD_Q arm, respectively. At the latchcircuit L5, the common mode of the clock signal should be about 1.1 V.In order to get a suitable common mode for the latch circuit L5 and alsofor the tri-state control circuit TS, the level shift circuits 86 areintroduced. The lower shift circuit 86 is needed in the PD_I arm toachieve a suitable clock common mode for the latch circuit L5. The levelshift circuit 86 in the upper PD_Q arm is introduced to keep a goodmatching. In order to generate two different common-mode voltages at theoutput, the two level shift circuits 86 are made slightly different. Ascan be gathered from FIGS. 19A and 19B, the common mode in the PD_I armis shifted lower than the common mode in the PD_Q arm. This is obviousdue to the additional resistor R2 in FIG. 19A. Furthermore, the levelshift circuits of FIGS. 19A and 19B introduce a significant gain suchthat the swing of the phase detection outputs PD_I and PD_Q is enhanced.

FIG. 20 shows a waveform diagram indicating respective signals at theinput of the amplifier circuit 84, at the input of the level shiftcircuit 86 and at the input of the latch circuit L5. As can be gatheredfrom FIG. 20, the swings of the phase detection outputs PD_I and PD_Qare firstly augmented by the amplifier circuit 84 and then againincreased by the level shifter circuits 86. Due to the modificationsshown in FIGS. 19A and 19B, the two level shifter circuits 86 havedifferent common mode levels at the output but they have exactly thesame gain to the differential arrangement.

FIG. 21 shows a circuit diagram of the tri-state control circuit TS ofFIGS. 3 and 16. When the frequency detection output of the latch circuitL5 is in a frequency-locked stage, the quadrature phase detection outputPD_Q will be active with a 50% duty cycle, this signal is seen as afrequency error. Therefore, the tri-state control circuit TS isintroduced to keep the coarse frequency detection loop completelysuppressed so that there will be no contribution from the coarse loop.The tri-state control circuit is controlled by the in-phase detectionoutput PD_I to suppress operation of the charge pump circuit 82 whenfrequency is in the locked state. In particular, in the frequency-lockedstate, the PD_I signal is positive and the transistors M1 and M2 arefully imbalanced. All bias current flows through the transistor M2 andcorresponds to the dump current I_(—)3State. Thus the I_(—)3Statecurrent or signal is used to suppress the charge pump as explainedearlier. This suppression of operation of the charge pump circuit 82 canbe based on a suppression of its tail current.

FIG. 22 shows a phase detector characteristic of the detectorarrangement as a simulation result for an input data rate of 10 Gb/s anda clock frequency of 10 GHz in the locked state. As can be seen in FIG.22, the phase detection output is not zero due to the intrinsic error.From the simulation result in FIG. 22 it can be seen that the phasedetector arrangement has a clear bang-bang characteristic and works asexpected with a periodic characteristic.

FIG. 23 shows a phase detection characteristic of the detectorarrangement with an input clock signal of 10 GHz and an input data bitrate changing from 6 Gb/s to 16 Gb/s. Thus, in FIG. 23, the horizontalaxis corresponds to the data bit rate, while in FIG. 22 the horizontalaxis indicates the time axis. The difference between the clock frequencyand the bit rate are interpreted frequency difference between −4 GHz and+6 GHz.

As can be gathered from FIG. 23, the phase detector arrangement cancorrectly react to clock-data frequency differences between ±3 GHz.Thus, the working frequency range of the phase detector arrangementshould be limited to a range between −3.5 GHz and +3.5 GHz. However, atthe positive side, the operating range continues monotonically up toabout 6 GHz.

FIG. 26 is a process flow diagram of a method of controlling a chargepump circuit used in a frequency detection arrangement in accordancewith an embodiment of the invention. At block 2602, first and secondcurrent sources respectively arranged in first and second differentialbranches of a differential input circuit of the charge pump circuit aremodulated. At block 2604, a tail current of the differential inputcircuit is controlled in response to a frequency-locked state of thefrequency detector arrangement.

Generally, it can be seen from the above description that the proposedphase-frequency detector arrangement can be used with frequencydifferences from −3.5 GHz up to +3.5 GHz. The advantage of thisimplementation resides in the possibility to pull-in large frequencyerrors. Also, the simplicity of the circuit and relatively low supplypower to a low power consumption and low fabrication costs. As anadvantage, the frequency detector gives a zero output to the charge pumpcircuit in the frequency locked state. Thus, the frequency variation iszero and the noise contribution from the frequency detector to thecharge pump circuit can be reduced significantly.

It is to be noted that the present invention is not restricted to theabove preferred embodiments but can be used in any frequency and/orphase detection arrangement with bang-bang characteristic. The preferredembodiments may thus vary within the scope of the attached claims.

1. A charge pump circuit for use in a frequency detector arrangement,said charge pump circuit comprising: a) a differential input circuithaving first and second differential branches for carrying first andsecond modulated currents; b) first and second current mirror circuits(CM) respectively provided in said first and second differentialbranches and arranged to copy the modulated current of one differentialbranch into the respective other differential branch; and c) controlmeans for controlling a tail current of said differential input circuitin response to a frequency-locked state of said frequency detectorarrangement.
 2. A charge pump circuit according to claim 1, furthercomprising a common mode rejection means (C) for comparing a common-modevoltage at the output of said charge pump circuit with a referencevoltage (V_(CM)), and for controlling said first and second currentsources based on the comparison result.
 3. A charge pump circuitaccording to claim 1, wherein said control means comprises switchingmeans (M1, M2) for switching said tail current in response to a controlsignal indicating said frequency-locked state.
 4. A method ofcontrolling a charge pump circuit used in a frequency detectionarrangement, said method comprising the steps of: a) modulating firstand second current sources respectively arranged in first and seconddifferential branches of a differential input circuit of said chargepump circuit; and b) controlling a tail current of said differentialinput circuit in response to a frequency-locked state of said frequencydetector arrangement.